There are several multiple access modulation techniques for facilitating communications in which a large number of mobile user are present. These techniques include time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA).
In TDMA radio telecommunication systems, the communication on the radio path is carried out time-divisionally in successive TDMA frames each of which consists of several time slots. A short information packet is transmitted in each time slot as a radio frequency burst having a finite duration and consisting of a group of modulated bits. Time slots are mainly used for carrying control channels and traffic channels. Speech and data are transmitted on traffic channels. Signalling between a base station and mobile subscriber stations is carried out on the control channels. One example of a TDMA radio system is a pan-European mobile communication system GSM (Global System for Mobile Communications).
CDMA is a modulation and multiple access scheme based on spread spectrum communication. Unlike FDMA or TDMA, in CDMA a large number of CDMA signals (users) simultaneously share the same wide band radio channel, typically 1.25 MHz. Pseudorandom noise (PN) binary codes, so called spreading codes, are used to distinguish between different CDMA signals, i.e traffic channels on a wide band radio channel. A separate spreading code is used over each connection between a base station and a subscriber terminal. In other words, the narrow-band data signal of the user is conventionally multiplied by the dedicated spreading code and thereby spread in bandwidth to the relatively wide band radio channel. The signals of the users can be distinguished from one another in the receivers on the basis of the unique spreading code of each connection, by using a correlator which accepts only a signal energy from the selected spreading code and despreads its spectrum into a narrow-band signal. The other users' signals, whose spreading codes do not match one another or that in use by the exemplary user now being discusses, are not despread in bandwidth and as a result, contribute only to the noise and represent a self-interference generated by the system. The spreading codes of the system are preferably selected in such a way that the codes used in each system cell are mutually orthogonal, i.e. they do not correlate with each other. Thus, in the CDMA systems, the spreading code unique to each user or user' signal provides a traffic channel in a similar sense as a time slot in the TDMA systems. CDMA is described in more detail in the document: "An overview of the application of code division multiple access (CDMA) to digital cellular systems and personal cellular networks", Qualcomm Incorporated, 1992, USA, (Document Number EX60-10010).
In traditional TDMA and CDMA mobile communications systems, the maximum data rate at the radio interface is relatively low.
In traditional mobile systems each mobile station is allocated a single traffic channel for data or speech transmission. Thus, in the GSM system, for example, a common carrier wave can carry up to eight parallel connections to different mobile stations. The maximum data transfer rate on one traffic channel is limited to a relatively low value according to the available bandwidth and channel coding and error correction used in the transmission, e.g. 9.6 kbit/s or 12 kbit/s in the GSM system. In the GSM system, a half-rate (max. 4.8 kbit/s) traffic channel can also be selected for low speech coding rates. A half-rate traffic channel is established when a mobile station operates in a specific time slot only in every second frame, that is, at half rate. Another mobile station operates in every second frame in the same time slot. Thus, in terms of the number of subscribers, the capacity of the system can be doubled, that is, as many as 16 mobile stations can operate on the same carrier at the same time.
In recent years, the need for high-speed data services has grown significantly. For example, transmission rates of 64 kbit/s or higher are needed for utilizing the circuit switched digital data services of the ISDN (Integrated Services Digital Network). Higher transmission rates, such as 14.4 kbit/s, are needed for data services of the public switched telephone network PSTN, such as modem and class G3 telefaxes. Mobile video service is one area of growth in mobile data transmission that requires higher transmission rates than 9.6 kbit/s. Security surveillance by cameras and video databases are examples of these services. The minimum data rate in video transmission may be 16 or 32 kbit/s, for example.
The transmission rates of present mobile communication networks are not, however, sufficient for satisfying these new requirements.
One solution, disclosed in international patent application W095/31878, is to allocate to one high-speed data transmission two or more parallel traffic channels on the radio path. A high-speed data signal is divided into these parallel traffic channels at the transmission end for the transmission over the radio path and it is reassembled at the reception end. Thus, data transmission services are provided in which, depending on the number of the allocated traffic channels, the transmission rate may be up to eight times higher in comparison with the standard transmission rate. For example, in the GSM system two parallel traffic channels provide a transmission rate of 2.times.9.6 kbit/s, which is sufficient for a modem or a telefax of 14.4 kbit/s. Six parallel traffic channels enable a transmission rate of 64 kbit/s.
In using parallel traffic channels, the problems are how to divide the data flow between parallel transparent traffic channels and how to synchronize the reassembling of data received from the parallel traffic channels.